Conventionally, since a plurality of signals which are transmitted in the same frequency band by using a unique spread code from each terminal unit are mixed and then received in CDMA mobile communication system, a system performance becomes deteriorated by a multiple access interference generated when despreading a received signal. In order to reduce this multiple access interference of the CDMA mobile communication system, a group-wise interference cancellation method has been employed. There are two kinds of the group-wise interference cancellation methods, that is, GSIC (Group-wise Successive Interference Cancellation) method and GPIC (Group-wise Parallel Interference Cancellation) method.
The GSIC (Group-wise Successive Interference Cancellation) method groups a plurality of signals according to the signal power, sequentially selects the signals one by one in order of the magnitude of the signal power, and thus performs the interference cancellation. As the GPIC (Group-wise Parallel Interference Cancellation) method, GW-PIC (Group-wise Weighted Parallel Interference Cancellation) method for giving a weight value to every group has been employed.
The aforementioned group-wise interference cancellation method makes a signal group in consideration of either a power of a transmission signal or the data speed, or makes a signal group by using the received signal. In the first case for making a signal group in consideration of either the power of a transmission signal or the data speed, the signal rank and the signal group are previously determined when transmitting signals. However, in a multipath fading channel environment, although a signal is very intensive at the time of a signal transmission, the power of the signal may be very attenuated due to an influence of fading and multipath when receiving the signal. Therefore, if such a signal participates in an interference cancellation, it may increase the noise to deteriorate the system performance. In addition, if the signal grouping is made by using the maximal ratio combiner output of the received signal of each finger, grouping is performed for each detector.
A typical prior technique for overcoming the above problems of the aforementioned group-wise interference cancellation methods is disclosed in Korean Patent Registration No. 255565, entitled “multiple mode subtraction-type noise cancellation method and apparatus in multiple mode asynchronous multipath channel of CDMA system”, filed by the applicant of this application, describing a multiple mode interference cancellation method in detail.
Such a multiple mode interference cancellation method groups a plurality of signals according to the power of the signal received at each finger, first selects a group having the greatest signal power, then sequentially selects a plurality of groups in the order of the magnitude of signal power, performs an interference cancellation by subtracting regeneration signals from the selected group, and performs an adding step in the next interference cancellation step. These steps are repeated for each interference cancellation step.
Since the above multiple mode interference cancellation method divides groups according to the power of the received signal of each finger, relatively accurate grouping can be achieved. However, in case of multiple rate transmission in which the bit length is different in each of signals, a signal with a high instantaneous bit energy has a higher reliability as compared to a signal received with a high instantaneous power. Therefore, considering only the power of the received signal causes a problem that makes the regeneration signal for subtraction inaccurate.